In analyzing a sample of double-stranded DNA, it has been determined that the molar amount of adenosine is 20%. Given this information, what is the content of cytidine?
A) 10
B) 20
C) 30
D) 40
E) 60

Question

Quizplus · Accepted Answer

In double-stranded DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). If adenosine (A) is 20%, then thymine (T) is also 20%. This leaves 60% for cytosine (C) and guanine (G) combined. Since C and G are equal, cytidine (C) is 30%.  The molar ratio of adenosine in any molecule of double-stranded DNA will be equivalent to that of thymidine, since these two nucleotides hydrogen bond to form base-pairs. Therefore, the total amount of the DNA accounted for in A-T base-pairs would be 40%. Since guanosine and cytidine hydrogen bond to form base-pairs, they too contribute an equivalent molar ratio in double-stranded DNA. In doublestranded DNA with 40% A-T composition, the molar ratio of cytidine would be half of the remaining 60% of the DNA, or 30%.

In Analyzing a Sample of Double-Stranded DNA, It Has Been